1. Field of the Invention
The invention relates to an image over-driving device and an image over-driving controlling method for a liquid crystal display (LCD), and more particularly to an image over-driving device and an image over-driving controlling method for avoiding overshot effect for an LCD.
2. Description of the Related Art
An LCD comprises an array of pixels. In each pixel, a liquid crystal is controlled by a cross voltage thereof to change a transmittance ratio, and a desired gray level is represented according to the transmittance ratio of the liquid crystal.
FIG. 1 is a schematic diagram of a conventional LCD panel and peripheral driving devices thereof. As shown in FIG. 1, a display array 1 is formed by interlacing data electrodes D1 to Dm and gate electrodes G1 to Gn. The interlaced data electrode and gate electrode are arranged to control one display unit. For example, the interlaced data electrode D1 and gate electrode G1 control a display unit 14. Referring to FIG. 1, each display unit comprises a thin film transistor (TFT) (Q11-Q1m, Q21-Q2m . . . Qn1-Qnm) for controlling the data input and a storage capacitor (C11-C1m, C21-C2m . . . Cn1-Cnm). A gate and a drain of the TFT are respectively coupled to a gate electrode (G1-Gn) and a data electrode (D1-Dm). Through a scan signal on the gate electrode (G1-Gn), the TFTs on the same row (that is on the same gate electrode) can be turned on/off, thereby controlling whether video signals on the data electrodes (D-Dm) are written into the corresponding pixel units.
As for peripheral driving devices also shown in FIG. 1, a gate driver 10 provides scan signals to the gate electrode G1-Gn according to a predetermined scan order. When one gate electrode carries the scan signal, the TFTs within the pixel units on the row or on the gate electrode are turned on. When one gate electrode is selected, a data driver 12 provides video signals to the pixel units on the gate electrode through the data electrodes D1-Dm according to image data that is prepared, but not yet displayed. A single frame is displayed each time the scan driver 10 finishes scanning all of the n rows. Therefore, the object of displaying images is achieved by repeatedly scanning scan lines and outputting video signals.
A timing controller 16 receives RGB color signals and timing signals for the display controlling, such as a vertical synchronization signal, a horizontal synchronization signal, a clock signal, and a data enable signal, from an external graphic controller or graphic card. According to the timing signals, the timing controller 16 outputs a gate-electrode control signal to the gate driver 10 and outputs the RGB color signals and data control signals to the data driver 10 for the display controlling.
In order to accelerate polarity change of liquid crystal molecules (referred to liquid crystal display units hereinafter) and the speed in which the liquid crystal display units reach the target gray level, a conventional timing controller is required to adjust voltage provided to the liquid crystal display units by using an over-driving method. An 8-bit panel which can display 256 (28) gray levels is given as an example in the following, wherein the lower gray level represents a darker image, while the higher gray level represents a lighter image. When an image displayed by a liquid crystal display unit is changed from the 0 gray level to the 230 gray level, a conventional timing controller provides greater cross voltage (for example, a voltage corresponding to the 250 gray level) to the liquid crystal display unit, thereby achieving the object of accelerating the gray-level change.
Since response of a liquid crystal display unit of an AV-type LCD is longer when it displays images with low gray-levels, efficacy of the over-driving procedure has to be enhanced. However, for the conventional over-driving method, image quality is degraded due to the overshot effect when size of the displayed object is too small or speed thereof is too fast.
FIG. 2A shows movement of a displayed object by frames, and a displayed object with a low gray-level is given as an example. As shown in FIG. 2A, a displayed object 20 moves along the direction of the arrow 22. FIG. 2A shows the positions of the displayed object 20 in the (N−1)th frame, the Nth frame, and the (N+1)th frame. FIG. 2B shows gray-level change of the liquid crystal display unit displaying the object 20. The target gray level of the liquid crystal display unit is a high gray level (for example 230 gray level) in the (N−1)th frame, a low gray level (for example 5 gray level in the Nth frame, and a high gray level (for example 230 gray level) in the (N+1)th frame. In FIG. 2B, a solid line represents the target gray level, and the dotted line represents the actual gray level of the liquid crystal display unit.
As shown in FIG. 2B, due to the limitation from the response time of the liquid crystal display unit, the actual gray level of the liquid crystal display unit in the Nth frame is not decreased to the low target gray level. In the (N+1)th frame, an over-driving cross voltage is provided to the liquid crystal display unit according to the difference between the target gray levels of the Nth frame and the (N+1)th frame by the conventional over-driving method. Since the actual gray level of the Nth frame is higher than the target gray level thereof, when an over-driving cross voltage is provided to the liquid crystal display unit according to the difference between the target gray levels of the Nth frame and the (N+1)th frame by the conventional over-driving method, the actual gray level in the initial period of the (N+1) the frame is significantly over the target gray level of the (N+1)th frame, resulting in overshot effect.